Interpretive Summary: Barley yellow dwarf virus (BYDV) infects all cereal crops and is distributed worldwide. The disease is caused by several different, but related, types of viruses. The virus is spread by aphids which feed on infected plants, then move to a new host plant. BYDV is responsible for yearly crop losses estimated at between 2 to 10% of the total United States grain crop; however periodic outbreaks of the disease cause losses that exceed these estimates. Epidemics of the disease result from the complex interaction of aphid vector populations, susceptible host plants, specific environmental conditions and the type of virus. Variability in the ability of viruses to cause disease has not received attention as a factor in the severity of disease outbreaks because few tools exist to distinguish among the viruses that cause barley yellow dwarf. This study examined the genetic variability of viruses collected from the field and their differential ability to cause disease. We developed laboratory tools capable of distinguishing several different viruses at the molecular level and found that some of these caused more severe disease on crop hosts than others. Most importantly, we identified a virus collected from the field that caused a 75% reduction in the grain weight of an oat cultivar that is widely grown because of it's resistance to BYDV. The occurrence of viruses that overcome plant resistance could be an important factor in outbreaks of disease epidemics. The methods we developed in this study can be used to diagnose viruses that have the potential to cause significant crop losses.

Technical Abstract:
Several barley yellow dwarf virus isolates collected from viriliferous aphids in upstate New York were identified as "PAV-like" based on their aphid transmission and their reaction with PAV-specific polyclonal antisera. Four of six isolates examined were distinguished from the NY- PAV isolate by their failure to react with a NY-PAV specific monoclonal antibody in ELISA and by restriction fragment length polymorphisms (RFLP) of polymerase chain reaction amplified viral sequences. The coat protein coding sequence of one of these four isolates (designated as isolate 129), which was distinguished by a unique RFLP, was cloned and sequenced. The deduced amino acid sequence of the coat protein from this isolate shares 87% sequence identity with that of NY-PAV, which is lower than the 97% identity reported among the three other PAV isolates that have been sequenced to date. Comparisons of the transmission efficiency of these two isolates using two aphid species indicate that this isolate was transmitted as efficiently as the NY-PAV isolate. Isolate 129 caused more severe symptoms than NY-PAV when inoculated to a variety of oat genotypes. Most importantly, the growth and grain yield of Ogle oat, a variety considered tolerant to infection by "PAV-like" isolates, was significantly reduced when infected with isolate 129. These results show that field isolates within the PAV serotype can be distinguished on the basis of RFLP analysis, and serological and biological differences. Furthermore, differences in the virulence of "PAV-like isolates on different host genotypes may contribute to the epidemiology of barley yellow dwarf disease.